1. Field of the Invention
The present invention relates to an antenna device having an antenna for transmitting and receiving radio signals and also relates to an information terminal device equipped with the antenna device.
2. Description of the Related Art
With a dramatic development in the communication technology, a radio (wireless) system for an information terminal device, such as laptop computer and mobile telephone, has become popular explosively in recent years. Assuming that a laptop computer as the information terminal device has a multifunctional radio system on board, an antenna of the radio system is generally arranged on an upper part of a display unit sensitive to radio wave. Under such an arrangement, if the laptop computer has a transmitter-receiver circuit arranged in a main unit, then a distance from the antenna to the transmitter-receiver circuit (i.e. cables for connecting the antenna with the transmitter-receiver) is lengthened. As such an elongated cable is equivalent to a low pass filter (LPF), a high frequency signal in the gigahertz band, which is used by a radio device in a hopeful view of the information terminal device, becomes easy to be attenuated in the cable. That is, in view from the main unit of the radio device, the ratio of carrier signal to noise (C/N) in receiving the radio wave is reduced to deteriorate a noise figure (NF). Here, the noise figure designates a ratio of C/N at input to C/N at output.
As means for restricting the deterioration of N/F in the radio device, Japanese Patent Publication Laid-open No. 2006-33076 discloses an active antenna where both a signal reception line and a signal transmission line are arranged in the vicinity of the antenna and additionally, an amplifier is interposed in the signal reception line to amplify the radio signal on reception.
FIG. 1 is a bock diagram showing the schematic constitution of an active antenna. This active antenna includes an antenna 1, an active circuit 2 having a low noise amplifier 3 and a radio module 5. The active circuit 2 is connected to the radio module 5 by a signal line passing through a hinge 4. In transmitting, the active antenna produces a control voltage for switches in the active circuit 2 through the use of a transmission power on a single signal line connecting the active circuit 2 with the radio module 5, accomplishing a transmission in the route of avoiding the low noise amplifier 3. In receiving a radio signal, the active antenna changes the positions of the switches into the route of receiving the radio signal by a voltage superimposed on a signal on the signal line and further activates the low noise amplifier. In this way, the switching operation between transmission and reception of the radio signal can be accomplished by the above single signal line.
According to the active antenna mentioned above, since the radio signal on reception is amplified by the low noise amplifier 3 before being attenuated by the signal line, it is possible to suppress a reduction of C/N in spite of attenuation of the radio signal via the signal line. Consequently, it is possible to prevent deterioration of NF of the radio unit.
FIG. 2 is a view showing the constitution of a laptop computer 10. The laptop computer 10 comprises a display unit 12, hinge parts 14 and main unit 16. As shown in the figure, the display unit 12 is connected to the main unit 16 by the hinge parts 14 for effecting a relative rotation therebetween.
In the general laptop computer 10 constructed above, the antenna 1 of FIG. 1 is generally arranged in a sensitive upper part 11 of the display unit 12. The radio module 5 is arranged inside the main unit 16. Therefore, the cables connecting the active circuit 2 with the radio module 5 have to be wired so as to pass through the interior of the hinge parts 14.
Further, as the radio signal received by the antenna 1 in the upper part 11 would be attenuated in the course of transmitting into the radio module 5 through the cables, the active circuit 2 for amplifying the radio signal should be arranged inside the display unit 12, as similar to the antenna 1.
Thus, due to an arrangement behind a liquid crystal part forming the display unit 12, the active circuit 2 is required to be thin. Meanwhile, the active circuit 2 has to be provided with an electromagnetic shield structure for suppressing an influence of electromagnetic waves. If the active circuit 2 is provided with no electromagnetic shield structure, there is a possibility of problems in radio communication since the active circuit 2 is under the influence of noise produced from the liquid crystal part. Additionally, there may be produced problems in liquid crystal displaying since the distribution of lights in the liquid crystal part is disturbed by noise produced from the active circuit 2. After all, it is necessary that the active circuit 2 is thinned (low height) to have an electromagnetic shield structure.
FIG. 3 illustrates a conventional electromagnetic shield structure for a high frequency circuit, for example, the above active circuit 2. The illustrated shield structure comprises a substrate 24 having a shield frame 22 and a shield cover 20. The shield frame 22 made of metal is soldered onto the substrate 24 so as to realize an electrical connection between a ground layer (not shown) in the substrate 24 and the frame 22. In connection, the shield frame 22 is arranged so as to surround a not-shown circuit as a noise source. The shield cover 20 as a lid is also made of metal and overlaid on the shield frame 22. Again, the shield cover 20 is provided, on its side faces, with a plurality of emboss parts 21 at predetermined intervals. The emboss parts 21 constitute electrical contacts between the shield cover 20 and the shield frame 22. Each interval between the adjoining emboss parts 21 is smaller than a half wavelength of an electromagnetic wave as an object to be shielded. With this arrangement of the emboss parts 21, the circuit(s) surrounded by the shield frame 22 can be protected from the outside electromagnetic wave and additionally, an electromagnetic wave originating in the inside circuit(s) can be prevented from leaking out.
FIGS. 4A and 4B are sectional views showing the above-mentioned electromagnetic shield structure. As shown in FIG. 4A, the shield frame 22 is soldered onto the substrate so as to be electrically connected to the ground layer 28 embedded in the substrate 24. The above high frequency circuit 26 is mounted on the substrate 24. As shown in FIG. 4B, the high frequency circuit 26 is electromagnetically shielded by the shield cover 20 since it is overlaid on the shield frame 22 while engaging the emboss parts 21 in openings (or recesses) 23 formed in the shield frame 22. In this way, by the shield structure comprising the shield cover 20, the shield frame 22, the emboss parts 21 and the ground layer 28, it is possible to protect the high frequency circuit 26 from the outside electromagnetic wave and also possible to prevent the electromagnetic wave in the circuit 26 from leaking out.